I. Field of the Invention
This invention relates to a biaxially oriented polyester film. More particularly, this invention relates to a biaxially oriented polyester film suited as a substrate for magnetic recording media, with which a high quality picture can be obtained and which gives an excellent abrasion resistance.
II. Description of the Related Art
As a substrate for magnetic recording media such as video tapes and audio tapes, biaxially oriented polyester films are conventionally used. To give good slipping to the recording media, the conventional substrates contain particles by which protrusions are formed in a surface of the film. The protrusions reduce the coefficient of friction of the film so as to give good slipping to the media.
U.S. Pat. No. 4,720,412 discloses a biaxially oriented film suited as the substrate for magnetic recording media, of which protrusions have specific height and number. The film disclosed in this USP has a drawback that the surface of the film is likely to be scratched by a roll or a guide which contacts the film in the process of applying the magnetic layer, in calendering process or in the process of dubbing a finished video tape or the like to manufacture a soft tape. Since these processes are conducted with higher speed recently than before, the tendency to be scratched is becoming a big problem. Further, conventional magnetic recording tapes have a disadvantage in that when dubbed, the quality of the picture is degraded, i.e., the S/N ratio (signal/noise ratio) of the dubbed tape is unsatisfactorily low.